1. Field of the Invention
The present invention relates to a Bipolar Junction Transistor device, and more specifically, to a method of fabricating self-aligned Bipolar Junction Transistor with expitaxy base.
2. Description of the Prior Art
Bipolar Junction Transistor (BJT) is a transistor device using electrons and holes carrier to transport currents. Bipolar Junction Transistor has three electrodes, emitter, base and collector. The function of the Bipolar Junction Transistor is to enlarge a input current signal, and therefore the Bipolar Junction Transistor can be transferring current amplifier of voltage signal.
The speed of the Bipolar Junction Transistor is faster than MOSFET. Nevertheless, operating dissipation energy of the Bipolar Junction Transistor is higher than the MOSFET, and dissipation heat removal is a barrier for the Bipolar Junction Transistor application in devices integration increasing. In some special application, the Bipolar Junction Transistor is still needed. Moreover, an applied product is combined CMOS with BJT to form BiCMOS device. The BiCMOS device is formed through considering the advantage of faster speed of BJT and lower dissipation energy of CMOS.
The BiCMOS device includes BJT and CMOS these two structures, wherein the BJT device is formed to provide high speed and high current driving in a circuit and CMOS device is formed to provide high integration and low dissipation energy in the circuit. Therefore, the BiCMOS device can provide many good functions, for example, higher speed, lower noise and low power.
Besides, a newer BJT application device is Hetro-junction Bipolar Transistor (HBT). The HBT device is formed using GaAs material, and it can apply to microwave. The emitter, base, and collector configuration of the HBT is arranged vertically, and the electric current in channel is also in vertical direction. The structure characteristic can reach extremely power density. That is, in the same output power, the die size of HBT can be permitted smaller, and operated in single power source.
Furthermore, the HBT have preferable properties in linear response and power efficiency, and therefore, the HBT has become a key device for mobile phone and personal communication service.
The BJT and HBT device have become more popular and have been widely used. However, traditional fabricating method of BJT has some drawbacks; for example, extrinsic base resistance (Rbbxe2x80x2) is still affected by expitaxy polysilicon thickness variation on oxide/SiN. Moreover, self-aligned collector implant (SIC) will be implanted through expitaxy film, and this will degrade the expitaxy film quality. If the collector implant is performed before formation of expitaxy film, CB (collector/base) junction will be too high to reduce the speed of electrons and holes carrier.
For advanced generation BJT making process, the emitter window width is not enough for CD/AA margin. For these reasons, it is desired to find a BJT making method to solve above issues.
Owing to traditional formation of BJT""s drawbacks in the prior art, the present invention discloses a method of fabricating self-aligned Bipolar Junction Transistor with expitaxy base.
The steps of the present invention include forming a shallow trench isolation structure in a substrate. An oxide layer is formed on the substrate. Subsequently, a polysilicon layer is next formed on the oxide layer, and the polysilicon layer has first type ion. Successively, a polysilicon layer is patterned on the oxide layer. The next step is to perform a second type ion implantation, thereby forming a collector region in the substrate and below the emitter window. The oxide layer is removed inside the emitter window. An expitaxy base is then formed on the polysilicon layer and substrate, thereby forming base region on the collector region, wherein the expitaxy base has the first type ion. After the expitaxy base is formed, a dielectric layer is formed over the expitaxy base. Next, the dielectric layer is etched to form inner spacer on sidewalls of the expitaxy base inside the emitter window. A second polysilicon layer is formed over the expitaxy base and the emitter window, wherein the second polysilicon layer has the second type ion. Finally, an etching process is introduced to etch the second polysilicon layer to form emitter plug. That is self-aligned to the emitter window.